It is well-known that the use of multiple antennas at the transmitter and/or the receiver can significantly boost the performance of a wireless system. Such multiple-input-multiple-output (MIMO) antenna configurations have the potential of both improving data rates as well as increasing the diversity.
Precoding is a popular multi-antenna technique for improving the performance of a MIMO system by transforming the information carrying transmit vector so that it better fits the channel conditions. This can be done based on instantaneous channel information or completely without channel information or some combination thereof. Often, precoding is implemented as performing a linear transformation on the information carrying vector prior to transmission. Such linear transformation is usually represented by a matrix. Precoding is an integral part of LTE, Long Term Evolution, as well of WCDMA, Wideband Code Division Multiple Access.
There are two basic flavors or types of precoding—codebook based and non-codebook based. Codebook based precoding means that the precoding matrix implementing the linear transformation is selected from a countable and typically finite set of candidate matrices. The mentioned set constitutes the codebook. Channel dependent codebook based precoding can be seen as a form of channel quantization since typically a set of channel realizations map to a certain precoding element. Non-codebook based precoding, on the other hand, does not involve any quantization, the precoding element can thus for example be a continuous function of the channel matrix.
Beamforming is a special case of the more general notion of precoding where a single information carrying symbol stream is multiplied by a channel dependent vector that adjusts the phase of the signal on each transmit antenna so that coherent addition of the transmit signals is obtained at the receiver side. This provides diversity as well as increases the signal-to-noise-ratio, SNR.
The precoder element to use may need to be signaled by means of feedback signaling and/or signaling of chosen precoder element in forward link. The feedback signaling can be viewed as a way for the receiver to provide channel information to the transmitter.
Several different approaches are common for forward link signaling. Explicitly signaling the precoder element index in forward link is possible. It is also possible to signal it implicitly using precoded pilots/reference symbols, RS, that together with non-precoded reference symbols can be used at the receiver to determine the used precoder element. Another possibility is to use precoded reference symbols also for the demodulation of the data, so-called dedicated RS, and from the receiver's point of view it will absorb the precoder element into the effective channel.
For maximum performance, the precoding element should be chosen to match the effective channel including transmit and receive filters, channel responses of antenna cables and of course the actual propagation channel. If the effective channel varies over the bandwidth allocated to communication, then there is a need to adapt the precoding over frequency as well to get the best possible match with the frequency-selective channel. Naturally, this affects the signaling of precoder elements so that a finer frequency granularity of the feedback and forward link signaling may be needed. If dedicated RS is used, it reduces the coherence bandwidth of the effective channel, which means that channel estimation procedures at the receiver side may have less data to average over and thus negatively affect the estimation accuracy.